Hot-dip galvanized steel sheets (galvanized steel sheets) are used in wide ranging applications such as automobiles, house-hold appliances, and constructional materials. Among them, galvannealed steel sheets (alloyed galvanized steel sheets) excel in corrosion resistance and spot weldability and are thereby widely used as materials for automobiles. Such galvannealed steel sheets are prepared by subjecting a galvanized steel sheet to a heat treatment to alloy a galvanized layer and a base steel sheet (steel sheet before hot-dip galvanization).
Base steel sheets for use in automobiles should have higher strengths and have smaller thicknesses, because automobiles should be reduced in body weight to improve fuel efficiency and should have higher strengths to improve collision safety. However, regular base steel sheets, if designed to have higher strengths, show inferior ductility. To avoid this, demands have been made to provide base steel sheets having strength and ductility in good balance.
To further improve both strength and ductility while maintaining good balance between them, the addition of silicon (Si) and/or manganese (Mn) may be performed. However, the addition of Si and/or Mn may significantly adversely affect plating wettability and alloying performance, because these elements are oxidizable elements and are thereby oxidized during annealing performed before hot-dip galvanization. Such poor wettability may cause uneven deposition of a plated layer on the surface of the base steel sheet and thereby cause unplated portions. The resulting plated layer, if deposited, may have a wavy “ripple” pattern on the surface and have poor appearance. The defective plating often causes uneven alloying, thereby impedes the control of alloying conditions, and impedes stable production of the galvannealed steel sheets.
In addition, the generation of defective plating (generation of unplated portions and generation of a ripple pattern) and the generation of uneven alloying cause inferior powdering resistance, which causes the plated layer to be peeled off from the base steel sheet in processing of the part, resulting in poor surface appearance. Techniques for solving these problems are disclosed in Patent Literature (PTL) 1 to 5.
PTL 1 discloses a technique of improving wettability between a base steel sheet and a galvanized layer by removing the surface layer of the annealed base steel sheet through dry etching prior to the immersion in a galvanizing bath. Such improved wettability prevents the generation of defective plating and uneven alloying. PTL 2 discloses a technique of applying a sulfur-containing ammonium salt to the surface of a high-tensile-strength steel sheet containing manganese (Mn), subjecting the steel sheet to a heat treatment, and subjecting the heat-treated steel sheet to a hot-dip galvanization. PTL 3 discloses a technique for improving platability (the property of galvannealed coating) by controlling the thermal hystereses before and after hot-dip galvanization to thereby improve coating adhesion in a width direction of a galvannealed steel sheet using a steel containing silicon (Si) and phosphorus (P) in high contents, thus avoiding uneven plating. PTL 4 discloses a technique of annealing a high-tensile-strength steel sheet in a continuous annealing furnace having a heating zone of clean heating furnace type or direct heating furnace type, removing 70% or more of a surface enriched layer typically of Si, Mn, and Al through acid pickling, and performing hot-dip galvanization. PTL 5 discloses a technique of forming a reaction product in a surface layer of a steel sheet in an annealing process of the steel sheet to be plated, which reaction product is formed between an added element in the steel sheet and a component of the annealing atmosphere.
The techniques disclosed in PTL 1 to 4, however, require complicated production processes, because they require dry etching process before hot-dip galvanization, application of an ammonium salt to the steel sheet, control of the thermal hystereses before and after hot-dip galvanization, or control of the acid pickling conditions. Independently, a reaction product, if formed on the surface of the base steel sheet as in the technique disclosed in PTL 5, may contrarily cause defective plating and/or uneven alloying.
Incidentally, galvannealed steel sheets are superior in corrosion resistance to base steel sheets. However, the improvement in corrosion resistance significantly depends on the mass of coating of the galvanized layer, and the mass of coating has an upper ceiling. For further improving corrosion resistance, painting of the surface of the galvannealed layer or addition of Al or Mg to the galvannealed layer may be performed. However, the painting may cause defects and causes higher cost. Independently, the addition of Al or Mg to the galvannealed layer also inevitably causes higher cost. Even the corrosion resistance of the galvannealed layer itself is increased by the addition of Al or Mg, if the galvannealed layer is peeled off from the base steel sheet, the steel sheet shows significantly impaired corrosion resistance in the end.